In the field of damascene processing, any amount of copper oxide formed on an exposed copper surface before subsequent deposition of additional copper or a copper diffusion barrier layer is a problem. This is because copper oxide has a low conductivity, and thereby increases the resistance of copper lines formed during fabrication. Furthermore, copper oxide presents integration difficulties and provides failure opportunities during normal use, particularly when high current densities are present.
Nearly all metals, including copper, form a thin oxide layer upon exposure to air and ambient temperatures. Under these conditions, copper forms an oxide layer of approximately 10 to 20 Å thickness. At higher temperatures, oxidation will continue to occur due to oxygen diffusion through the oxide. The following equation approximately describes the thickness, in nanometers, of Cu2O formed on a Cu surface in oxygen:
            Cu      2        ⁢          O      ⁡              (        nm        )              =      A    ×          t              1        2              ⁢          ⅇ                        -          E                kT            where
      E    =          0.41      ⁢                          ⁢      eV        ,          ⁢      A    =                            (                      P                          1              ⁢                                                          ⁢              Torr                                )                    ×              (                  4          ×                      10            5                    ⁢                      nm                          min                                      )              ,t=time in minutes and P=oxygen pressure in Torr.
Because of oxide formation, many IC fabrication processes that involve oxidizing environments cannot be employed. Any time that a process might oxidize an exposed copper surface, significant difficulties result, mostly because of the aforementioned decrease in conductivity. Barrier materials, for example, are typically deposited by using thermal deposition, physical vapor deposition, and chemical vapor deposition in a reducing or oxidizing environment. Thus, barrier materials cannot be deposited on copper using CVD in oxidizing conditions in modern IC fabrication processes. Examples of potential barrier materials where deposition in an oxidizing environment is preferred include ruthenium, magnesium, magnesium oxide, iron, cobalt, and nickel.
What is needed, therefore, is a process for allowing deposition on copper in an oxidizing environment.